1. Technical Field of the Invention
The present invention relates to novel topically applicable UV-photoprotective compositions containing at least one normally photolabile dibenzoylmethane UV-screening compound and, as a photostabilizer therefor, an effective amount of at least one particular benzalmalonate silane.
This invention also relates to certain novel processes for the synthesis of said particular benzalmalonate silanes.
2. Description of the Prior Art
It is known to this art that light radiation of wavelengths ranging from 280 nm to 400 nm promote tanning of the human epidermis, and that radiation of wavelengths more particularly ranging from 280 to 320 nm, i.e., UV-B irradiation, causes erythemas and skin burns which are detrimental to the development of natural bronzing and tanning. For these reasons, as well as for aesthetic reasons, there is a constant demand for means to control this natural bronzing and thus control the coloration of the skin; it is thus advisable to filter such UV-B radiation from the skin.
It is also known to this art that UV-A irradiation, of wavelengths of from 320 to 400 nm, which promotes tanning of the skin, also adversely affects it, especially in the case of a sensitive skin or of a skin which is continuously exposed to solar radiation. The UV-A rays, in particular, cause a loss in the elasticity of the skin and the appearance of wrinkles, resulting in premature cutaneous aging. UV-A radiation promotes triggering of the erythematous reaction or amplifies this reaction in certain individuals and can even be the source of phototoxic or photoallergic reactions. Thus, for aesthetic and cosmetic reasons such as the preservation of the natural elasticity of the skin, for example, more and more individuals seek to control the effect of UV-A rays on their skin. It is thus desirable to also filter out UV-A radiation.
In this respect, one class of UV-A filters of particular interest comprises the dibenzoylmethane compounds, and especially 4-tert-butyl-4'-methoxydibenzoylmethane, which indeed have a great capacity for intrinsic absorption. These dibenzoylmethane compounds, which are per se well known as active filters in the UV-A region, are described, in particular, in FR-A-2,326,405 and FR-2,440,933, as well as in EP-A-0,114,607; 4-tert-butyl-4'-methoxydibenzoylmethane is also commercially available under the trademark "PARSOL 1789" by GIVAUDAN.
Unfortunately, the dibenzoylmethane derivatives are compounds which are relatively sensitive to ultraviolet radiation (especially UV-A irradiation), namely, they have an unfortunate tendency to degrade to a more or less rapid extent under the action of such radiation. Thus, this substantial lack of photochemical stability of dibenzoylmethane compounds with regard to ultraviolet radiation which by their very nature they are intended to be subjected, does not permit constant protection to be guaranteed during prolonged exposure to the sun, such that, in a restrictive fashion, repeated applications at regular and close intervals of time must be carried out by the user to provide an efficacious protection of the skin against UV rays.
The photostabilization of dibenzoylmethane compounds with respect to UV radiation remains a problem which to date has not satisfactorily been resolved.
Thus, EP-A-0,709,080 describes combining dibenzoylmethane derivatives with very specific benzalmalonate derivatives, namely, benzalmalonate siloxanes, in order to decrease the photoinstability of said dibenzoylmethane compounds. In particular, this '080 application mandates that the siloxane structure of these compounds is essential to provide satisfactory photostabilization (see page 10, line 22 of EP-A-0,709,080). However, these benzalmalonate siloxanes are molecules which are bulky and complex to synthesize.